One of the typical, but nevertheless important objectives in signature stacking is to produce signature bundles in which the signatures are aligned to a high degree of precision and in which the bundle is compressed sufficiently to remove as much air from between and among the signatures as is possible and to compress the folded signatures to form a bundle in which the top and bottom surfaces are as near to parallel as is practicable. For example, signatures of a tabloid form having a folded edge typically have a thickness adjacent the folded edge which is greater than the thickness adjacent the opposite parallel cut edge. The cumulative effect of this difference in thickness results in the formation of a bundle in which one side edge is significantly taller than the opposite side edge yielding an awkward bundle which is difficult to wrap and handle.
The standard technique utilized to correct the aforementioned problem typically consists of forming compensated bundles in which signature stacks which are typically of the same count are oriented at 180.degree. relative to the adjacent stack which substantially evenly distributes the folded edges by arranging them along opposite sides of the bundle. This is accomplished by depositing a stack of signatures upon a turntable in which the folded edges are all aligned on one side of the stack. The turntable is then rotated through 180.degree. preparatory to receiving the next stack of signatures, whereby the folded edges of the two stacks are arranged on opposite sides of the multi-stack bundle, thereby forming a compensated bundle.
The techniques presently employed to compress signatures to reduce the thickness of the folded edges comprise passing a stream of signatures being delivered to the stacker between squeeze rolls and/or compressing a stack of signatures between upper and lower compression members when the signatures reach a stacking bin.
Equipment is presently available which enables the delivery of signatures to the stacking equipment at significantly increased delivery rates thereby necessitating the need for high speed stacking equipment capable of operating at speeds commensurate with the delivery speeds of present day equipment.
Present day techniques for stack compression include the provision of either motor driven or hydraulically driven compression members which are normally retracted from the stacking region and are extended to a position above the last delivered stack. The platform supporting the stack of signatures is then raised to compress the stack between the compression members and the platform. The platform is then lowered, the compression members are then removed from the stacking region and the next stack is delivered to the stacking region, the compression members are returned to a position in the stacking region above the stack and the compression operation is again performed however, on both stacks deposited on the platform. This technique is repeated depending upon the number of stacks desired per bundle. The compression technique may be used alone or together with the compensation technique.
Conventional equipment requires the use of motor or hydraulic drives as well as control equipment to operate the compression members. In addition, in order to assure proper alignment of the signature bundle, it is typical to eliminate a final compression operation upon completion of the bundle since compression on bundle completion requires that the compression members be located at a distance above the platform sufficient to stack a fully completed bundle beneath the compression members and further due to the need to reduce cycle time in order to accommodate high speed signature delivery equipment.